Diindolo[2,3‐b:2′,3′‐f]pyrrolo[3,2‐b]pyrroles as Electron‐Rich, Ladder‐Type Fluorophores: Synthesis and Optical Properties

Abstract

Previously unknown ladder-type heteroacenes were synthesized in just two steps from aromatic amines, 2-nitrobenzaldehydes, and biacetyl. A recently discovered multicomponent process affording 1,4-dihydropyrrolo[3,2-b]pyrroles was followed by the Cadogan reaction, which resulted in simultaneous closure of two additional pyrrole rings. Rod-like compounds synthesized in this way were stable in the absence of light, and they were transformed into bis(tert-butoxycarbonyl) derivatives. The rigid herringbone-like structure resulted in a strong absorption in the λ=350-410 nm region and very strong violet and blue emission, which was bathochromically shifted relative to the emission of the parent 1,4-dihydropyrrolo[3,2-b]pyrroles. The optical properties of these compounds strongly suggest that the conjugation spanned the entire π system. The HOMO energy level was located at -4.6 to -5.1 eV.

Symmetry Breaking in Pyrrolo[3,2‐b]pyrroles: Synthesis, Solvatofluorochromism and Two‐photon Absorption

“…Design and synthesis Prior research on tetraaryl-pyrrolo [3,2-b]pyrroles [13,14,[17][18][19][20] has indicated that derivatives containing nitrophenyl substituents display especially pronounceds olvatofluorochromism effects. Therefore we decided to focus exclusively on compounds possessing this strongly electron-withdrawing group.…”

“…Reagent grade solvents (CH 2 Cl 2 ,h exane, toluene) were distilled prior to use. All reported 1 HNMR and 13…”

“…Notable exceptions have been pointed out by Terenziania nd co-workers [11] and recentlyt his phenomenon, i.e., symmetry breaking in the excited state has been intensively studied in variousl aboratories. [12] In this context, symmetry breakingi nq uadrupolar,c entrosymmetric pyrrolo [3,2-b]pyrroles became the focus of our interest, [13] especially after we have discovered that the 2,5-bis(4-nitrophenyl)-1,4-bis(4-octylphenyl)-1,4-dihydropyrrolo [3,2-b]pyrrole (5,s ee Scheme 1) displayed very strong solvatofluorochromism.I ts emission in cyclohexane was extremely strongw hile emission in moderately polar solvents such as CH 2 Cl 2 or THF was bathochromically shifted by 100 nm, with af luorescenceq uantum yield (F fl ) < 0.03. [14] This interesting discoveryp rompted us to perform an indepth investigationo ft he influence of size of the p-system and the degree of conjugation (by changing the dihedral angle between heterocyclic core and nitrobenzenes ubunit) on the linear and non-linear opticalp roperties of pyrrolo [3,2b]pyrroles possessing4-nitrophenylsubstituents.…”

Pyrrolo[3,2‐b]pyrroles—From Unprecedented Solvatofluorochromism to Two‐Photon Absorption

“…Recently we discovered a new and efficient methodology for the synthesis of pyrrolo[3,2- b ]pyrroles, [10] the most electron-rich heterocycle among aromatic two-ring systems. [11] It became tempting to utilize this scaffold in the construction of even stronger quadrupolar A-D-A chromophores and to study their two-photon absorption properties. In a preliminary study, we found that for a series of TetraAryl-1,4-dihydroPyrrolo[3,2- b ]Pyrroles (TAPPs), the two-photon absorption cross-sections reach values of σ 2PA ~700 GM at 700 nm, limited by the wavelength tuning range of the laser system.…”

C‐N and N‐N bond formation via Reductive Cyclization: Progress in Cadogan /Cadogan‐Sundberg Reactionǂ

“…Compounds 163 a‐b and 165 a were synthesized by treating compound 162 a‐b and 164 a with an excess of P(OEt) 3 in o‐DCB (Scheme ). In 2015, ladder type fluorophores 167 a‐e consisting of diindolo[2, 3‐ b :2’,3’‐f]pyrrolo[3, 2‐ b ]pyrrole moieties were prepared from 166 a‐e and explored for their optical properties (Scheme ) . Using Cadogan cyclization conditions along with Mo catalyst, bis(acetylacetonato) dioxo‐molybdenum (VI) (5 mol%) substituted pyrroles 168–173 (Scheme ) were prepared from nitrodienes …”